The setting of the end seals which prevent leakage of heat-exchange media at the end faces of a regenerative heat-exchange mass is a matter of importance. Sealing frames are borne on one part, here the rotating hoods, and these rotate over a planar end face, in this case on a static regenerative mass. If the sealing frames are held clear of the end face there will be leakage; if they press together there will be wear. To relieve the pressure at this interface the sealing frames have been isolated from their supporting structure by spring-loading so that only a small portion of the weight of the frame (on an upper end face) or only a small component of force due to the springs (on a lower end face) is used to urge the two into contact. This type of system was either completely uncontrolled (so that with variation in temperature, or temperature gradients across the regenerative mass, there was a risk of unduly large clearances developing or unduly large pressures being exerted) or was controlled by a mechanical drive which adjusted the spring tension, either as a result of manual initiation of the drive, or automatically in response to temperature sensed in the regenerator.